There are many known methods of measuring magnetic fields, including magnetometers based on moving and stationary coils, Hall effect, thin films, fluxgates, magnetic resonances, and superconducting devices. (For a review, see D. I. Gordon, R. E. Brown, and J. F. Haben, IEEE Trans. on Magnetics, Vol. May-8, 48(1972).) These have various limitations (e.g., high cost, necessity of cryogenic temperatures) and advantages (high sensitivity, etc.).
It is known to use light carrying optical fibers for detecting electromagnetic radiation. This principle has been used in the past in the practice application of detecting and measuring current in high voltage conductors. This method of detection usually involved passing a beam of polarized light through an optical fiber from one end to the other in the presence of a longitudinal magnetic field and measuring the extent or rotation (twist) of the polarized light plane. The extent of rotation is dependent upon the prevailing magnetic field. Such apparatus is disclosed in U.S. Pat. No. 3,936,742. Another example of this principle is discussed in U.S. Pat. No. 3,621,390 where the object is to measure a time-varied magnetic field where a light beam is split, passed through respective polarizers, which are circularly polarized in opposite direction, subjected to the Faraday effect and brought back into interference to produce an intensity-modulated light beam subject to detection. Such sensors, based on the Faraday effect, take advantage of the fact that right and left hand circular polarized light in a fiber undergo differential phase shifts when exposed to a magnetic field parallel to the direction of light propogation.
It has been proposed to use a magnetostrictive jacket around an optical fiber such that, when in the presence of a magnetic field, the jacket changes its dimensions so as to strain the fiber for causing an interferometry detectable phase shift in light passing through it when compared to light passing through a reference fiber.
The present invention proposes a much simpler arrangement in optical fibers for use in magnetometers for detecting geomagnetic anomalies or in antennas for detecting the magnetic field accompanying a time-varied electromagnetic radiation. It is known that metallic iron particles of spherical shape approximately 100 A in diameter can be precipitated in fused quartz by melting under highly reducing conditions and subsequent annealing in vacuo (J. Appl. Phys. 50 2402, 1978). It is further known that by suitable heating and stretching of glasses containing spherical precipitates, these spherical particles can be made to elongate into aligned arrays of needle-like shapes. (J. Non-Cryst. Solids 15 437, 1974). It is further recognized that slightly elongate particles of metallic iron having dimensions of approximately 50 to 200 A will have a single magnetic domain (J. Geophy. Res. 80, 252, 1975).